FILTRATION SYSTEM

Abstract: A filtration system for filtering dope of a solvent-spun product plant in which dope to be spun flows from a source of supply (10) through a series of filter assemblies (15, 21, 23) to jets (22(b)) of each spinning head (22). The filtration system comprises, in flow series, a first stage filter assembly (15) having filter elements with the finest pore size of the series (typically 20 mu ) and a final stage filter assembly (23), immediately upstream of the jets (22(b)) of each spinning head (22). The final stage filter assembly is of the coarsest pore size of the series of filters (typically 80 mu ) and is equal to, or less than, the size of the jet holes (typically 80 mu ). The filter media of intermediate filter assemblies (21) between the first (15) and final (23) stages increase in pore size as one progresses from the first filter assembly (15) to the final filter assemblies (23) along the flow path.

Description: Filtration Sy^- 0 ™ This invention relates to a filtration system for a cellulosic dope manufacturing process and plant. In the manufacture of solvent-spun products such as, for example, Tencel cellulose fibres (Tencel is a trade mark of Courtaulds Fibres Limited) , a dope comprising an aqueous solution of woodpulp and amine oxide, is supplied under pressure to a spinning head. The spinning head comprise a plurality of spinnerette jets which are typically 80μ or less in diameter if fibre is to be produced. The dope is extruded through the spinnerette jets into a spin bath where the solvent is leached out of the fibre and the fibre is washed by water. The fibres are collected, washed and dried whilst the waste aqueous amine oxide solution is recovered and returned to the process. The spinnerette jets for fibre are typically of the order of 80μ diameter and are carefully shaped and designed to optimise fibre production. In a modern fibre production plant, there can be as many as 200 spinning heads each with up to six spinnerette plates each of which could have as many as 7,000 jet holes of 80μ diameter. It is therefore essential to filter out particles or lumps in the dope which could block the spinnerette jet holes. The most obvious way to do this is to provide a series of filters of decreasing mesh sizes with the first filter of the series having the coarsest mesh and the most downstream filter, i.e. the one immediately in front of the spinnerette jets, having the smallest mesh size (less in size than the diameter of the spinnerette jet) . The finer the mesh, the more efficient will be the filter but the more likely it is to block up rapidly. It has been found to be impractical to achieve satisfactory filtration of the dope when using a series of filters arranged with decreasing mesh sizes because the finest filter immediately upstream of the filter blocks up easily and requires frequent changing and cleaning. Furthermore, because of the large number of filters that would be required upstream of the spinning heads, (one for each spinning head) , and the need to change them frequently, if they were to be of much smaller mesh than the diameter of the jets (80μ) it would be impossible to achieve a satisfactory design of filter, which is easy to clean. One aim of the present invention is to provide a filtration system for a cellulose dope manufacturing process which comprises a plurality of sets of filters in flow series and which are easy to clean without disrupting the flow of dope to the spinning heads of the process plant. According to one aspect of the present invention there is provided a dope filtration system for the manufacture of solvent-spun fibre or other elongated member in which dope to be spun is caused to flow through a plurality of filters from a source of supply to one or more extrusion heads which have a plurality of spinnerette jet holes of predetermined diameter, the filtration system comprising, in flow series, a plurality of filter assemblies of filter media of different pore sizes, the pore size of the filter media of a first of the filter assemblies being of the smallest size of all the filter assemblies, and the, or each, subsequent filter assembly of the series having a pore size which will filter out particles larger than that filtered out by the first of the filter assemblies, and the filter media of the final filter assembly of the series of filter assemblies having a pore size which will filter out particles at least equal to the size of the spinnerette jet holes. Preferably there are at least three filter assemblies between the source of supply of dope and the, or each, spinnerette head. Preferably the first filter assembly comprises at least two filters, connected in parallel in the flow path from the source of supply of dope to the, or each, spinnerette head, diverter valve means selectively operable so as to connect at least one of the filters in the flow path and disconnect at least one of the filters from the flow path, and means for adjusting the rate of flow of the dope through one or both of the filters of the first filter assembly so as to maintain a substantially constant flow of dope from the first filter assembly as selected filters of the first filter assembly are connected into, or disconnected from the flow path. Preferably the first of the filter assemblies comprises first and second filters connected in parallel between the source of supply of dope to be filtered and an outlet for the filtered dope, a first diverter valve located at an inlet to the first and second filters and selectively operable to divert dope to be filtered to a selected one or both of the filters, a variable speed pump means located upstream of the filters, a second diverter valve located at the outlet of the first and second filters and being selectively operable to receive flow of filtered dope from a selected one or both of the filters and to direct the filtered dope to the outlet for the filtered dope, a sensor means downstream of the filters for monitoring the flow of filtered dope and operable to produce a signal indicative of the flow of filtered dope through the first and second filters, and means responsive to the signal generated by the sensor means which is operable to control the speed of the pump means to maintain a predetermined flow of filtered dope through the first filter assembly. Preferably the first filter assembly comprises a plurality of tubes having a filter media made of sintered metal fibres matting mounted in a sealed vessel. Preferably the filter media of the first filter assembly have a pore size which will filter out particles in the range of 20μ to 30μ. Preferably the filter media of the final filter assembly has a pore size which will filter out particles of between 70μ to 80μ. In the case where there is one or more intermediate filter assembly the filter media of the one or more intermediate filter assemblies has a pore size which will filter out particles in the range of 30μ to 40μ. In a preferred embodiment of the invention,the diameter of the spinnerette jet holes is in the range of 70μ to 80μ, a first of the filter assemblies has a pore size which will filter out particles of between 20μ to 30μ, a second of the filter assemblies downstream of the first filter assembly has a pore size which will filter out particles of between 30μ to 40μ, and a third of the filter assemblies downstream of the second filter assembly has a filter media which has a pore size which will filter out particles less than 80μ. The present invention will now be further described, by way of example, with reference to the accompanying drawings in which:- Figure 1 shows schematically a plant for solvent spinning of cellulose fibres using a dope filtration system constructed in accordance with the present invention, Figure 2 shows in greater detail one filter element of the first filter assembly of the plant shown in Figure 1, and Figure 3 shows in greater detail the one of the final filter assemblies of the plant shown in Figure 1. Referring to Figure 1, dope, comprising woodpulp dissolved in an aqueous solution of 76% to 78% by weight of amine oxide (4-methyl morpholine-4-oxide) , is supplied from a tank 10 via a filmtruder 11 and tank 12 to the inlet of a feed pump 13 which supplies the dope to the first filter assembly 15 of a series of filters. The feed pump 13 is a variable speed pump which delivers a predetermined volume of dope at the outlet of the filter assembly 15 at a predetermined speed of the pump 13. Referring to Figure 2 each of the filter elements 16 of the filter assembly 15 comprises a tubular filter element mounted at one end in a header plate 17. Each of the tubes 16 is blanked off at the one end 18 and the cylindrical wall of the tube 16 comprises a porous filter media formed from sintered stainless steel fibre matting which is pleated longitudinally along the length of the tube. The header plate 17 is assembled into a filter vessel 19 (see Figure 1) to make a sealed chamber. The filter media of the elements 16 have a pore size of between 20μ to 30μ (preferably 20μ) and are required to filter out particles and lumps in the dope that are greater than 20μ. The dope which passes through the filter assembly 15, is pumped by a second pump 20 (called the spin feed pump) to a plurality of second filter assemblies 21 (only one of which is shown in detail) . Each second filter assembly 21 is of a similar construction as the filter assembly 15 but the sintered stainless steel fibre matting 2la used in each second filter assembly has a pore size of the order of 30μ to 40μ (preferably 40μ) which filters out particles or lumps of 40μ or greater. The dope which passes through each filter assembly 21 is supplied to a plurality of spinning heads 22. In a modern plant there are as many as 200 spinning heads, each of which has a plurality of spinnerette plates 22 (a) . Each spinnerette plate has as many as 7,000 trumpet shaped spinnerette jet holes 22(b) of typically 70μ to 80μ diameter formed in it. Immediately upstream of the jets 22(b) of each head 22 is a final filter assembly 23 which comprises a filter media made of two sintered stainless steel meshes 23 (a) supported on a foraminated plate 23 (b) . The pore size of the filters 23 is of the order of 70μ to 80μ and will filter out particles or lumps greater than the 70μ from the dope. The filters 15 and 21 are made of staple length stainless steel fibres sintered together to form a mat which is relatively thick (compared with the thickness of the meshes of filter 23) , and retain dirt more effectively than the filters 23. However the filters 15 have a more precise pore size and are effective at filtering out particles of 30μ. Spun fibre is extruded through the spinnerette jet holes into a spin bath 24 where the solvent is leached from the fibre and the fibre is washed with water. The spun fibre is collected and passes through a wash region 25 and a drying oven 26. Waste aqueous solution of the amine oxide from the spin bath 24 is returned to the tank 10 via a filter 27 and ion exchanger 28, and the water is evaporated using an evaporator 29. From the above, it will be seen that the filtration system of the present invention located between the dope supply 10 and each spinning head 22 comprises, in flow series, the first filter assembly 15, one of the second filter assemblies 21 and one of the third filter assemblies 23. Of the three filter assemblies 15, 21, 23 the filter media of each first filter assembly 15 is of the finest pore size (20μ) , and the filter media of each of the filters 23 of each third filter assembly are the coarsest pore size 70μ to 80μ. The filter media of each intermediate filter assembly 21 is of a pore size of the order of 40μ. This is the reverse of what one would normally expect. However it has been found to be advantageous because a small number of large capacity filters 15 of fine pore size can be used to filter the bulk of the dope, and can be easily changed without disrupting the flow of dope. On the other hand, the large number of filters 23, being of the coarsest pore size of the three filter assemblies 15, 21, 23, are less likely to block up and therefore require less frequent changing. Furthermore individual spinning heads 22 can be isolated j by the provision of isolation valves 40, to permit easy replacement of the filters 23 without disrupting the whole production of fibre. Similarly isolation valves 41 may be provided upstream of each filter assembly 21 to enable selected filters 21 to be removed and cleaned without disrupting flow of dope to the other filters 21. Referring to Figure 1, it can be seen that the first filter assembly 15 is shown as essentially two parallel banks of filters, 15a and 15b, only one of which is usually connected on-line at a time, except when changing over filters. For the following description it is assumed that the on-line filter is that shown as 15a and the other filter 15b is on "stand-by". On the outlet side of the dope feed pump 13 is a diverter valve 30 which is manually selectively variable from a first position where 100 per cent of the flow of dope passes through the filter 15a to a second position where 100 per cent of the flow dope is through the filter 15b. At intermediate positions of the valve 30, the flow is proportioned to both filters 15a and 15b. The outlet of the first filter assembly 15 is connected to a common inlet of the spin feed pump 20 via a second diverter valve 31. The spin feed pump 20 is a constant volume pump which runs at a constant speed to supply a uniform flow rate of dope to each spinnerette head 22. A pressure sensor and transducer 32 is provided at the inlet of the spin feed pump 20 and operates through a speed control circuit 33 to control the speed of the dope feed pump 13 in order to maintain a constant flow of dope to the inlet of the spin feed pump 20. In other words, as the on-line filter 15a starts to block up, the pressure tends to drop at the inlet to the pump 20 and the control circuit 33 operates to increase the speed of the dope feed pump 13 and thereby tends to restore the pressure and maintain the flow rate constant at the inlet to the pump 20. If the pressure drop across the filter 15a reaches a predetermined value which indicates the on-line filter 15a is too blocked to continue safely, the filters 15a and 15b are changed over in the following manner. The fresh clean filter elements 16 of the filter 15b are assembled in their respective vessel 19 and the diverter valve 30 is operated so as to divert some of the dope into the fresh clean filter 15b. A bleed valve 34 is operated to bleed all the air from the vessel 19 as it fills up. Opening the valve 30, to fill the spare filter 15b causes a slight pressure drop across the filter 15a which is sensed by the sensor and transducer 32. To compensate for this one can slow down the spin feed pumps 20 slightly so that production of spun product is decreased by the amount of solvent diverted to the fresh filter whilst maintaining the speed of the feed pump 13 constant. Alternatively, the pump 13 could be speeded up slightly by the control circuit 33 to compensate for the filling of the spare filter 15b and the spin feed-pumps 20 maintained at constant speed. When the filter 15b is completely filled with dope, and all air is expelled from its vessel 19, the bleed valve 34 is closed, the diverter valve 31 is opened gradually to connect filter 15b to the pumps 20 and at the same time the diverter valve 30 is operated so as to divert the supply of dope from the blocked filter 15a to the fresh filter 15b. As this is done, the speed of the pump 13 is adjusted under the control of the pressure control circuit 33 to maintain a constant flow rate of dope to the pumps 20. In the case where the pumps 20 were slowed down to compensate for the diversion of dope to filter 15b pumps 20 are speeded up to restore the flow of dope to the spinning heads to the previous production level. The blocked filter 15a is drained of its contents and can then be removed from the plant for cleaning. In the plant shown in Figure l, the second filter assemblies 21 are not duplicated and cannot be changed without isolating the spinning heads supplied by the filters 21. However, if desired, each second filter assembly 21 may comprise two filters similar to that shown for filter assembly 15 and similar valves (not shown) to the valves 30, 31 as used in the first filter assembly 15. These valves may be used and operated in the same way as the valves 30 and 31 so as to enable the flow of dope through a blocked filter 21 to be diverted through a second fresh filter 21. Here again, a pressure sensor and control circuit (not shown) could be provided to control the speed of each pump 20 so as to compensate for any changes in the pressure drop across the filters 21 when changing the filters 21.

Claims: CLAIMS 1. A dope filtration system for the manufacture of solvent-spun fibre or other elongate member in which dope to be spun is caused to flow through a plurality of filters from a source of supply to one or more spinning heads which have a plurality of spinnerette jet holes of predetermined diameter, characterised in that the filtration system comprises , in flow series, a plurality of filter assemblies (15, 21, 23) of filter media of different pore sizes, the pore size of the filter media of a first of the filter assemblies (15) being of the smallest size of all the filter assemblies, and the, or each, subsequent filter assembly (21, 23) of the series having a pore size which will filter out particles larger than that filtered out by the first of the filter assemblies (15) , and the filter media of the final filter assembly (23) of the series of filter assemblies having a pore size which will filter out particles at least equal to the size of the spinnerette jet holes (22b) . 2. A dope filtration system according to Claim 1, wherein there are at least three filter assemblies (15, 21, 23) between the source of supply (10, 11, 12) of dope and the, or each, spinnerette head (22) . 3. A dope filtration system according to Claim 1 or 2, wherein the first filter assembly (15) comprises at least two filters (15a, 15b) , connected in parallel in the flow path from the source (10, 11, 12) of supply of dope to the, or each, spinnerette head (22) , diverter valve means (30) selectively operable so as to connect at least one of the filters (15a, 15b) in the flow path and disconnect at least one of the filters (15a, 15b) from the flow path, and means for adjusting the rate of flow of the dope through one or both of the filters of the first filter assembly (15) so as to maintain a substantially constant flow of dope from the first filter assembly (15) as selected filters (15a, 15b) of the first filter assembly (15) are connected into, or disconnected from the flow path. 4. A dope filtration system according to Claim 1, 2 or 3, wherein the first of the filter assemblies (15) comprises first (15a) and second (15b) filters connected in parallel between the source of supply (10, 11, 12) of dope to be filtered and an outlet for the filtered dope, a first diverter valve (30) located at an inlet to the first (15a) and second (15b) filters and selectively operable to divert dope to be filtered to a selected one or both of the filters (15a, 15b) , a variable speed pump means (13) located upstream of the filters, a second diverter valve (31) located at the outlet of the first and second filters (15a, 15b) and being selectively operable to receive flow of filtered dope from a selected one or both of the filters and to direct the filtered dope to the outlet for the filtered dope, a sensor means (32) downstream of the filters (15a, 15b) for monitoring the flow of filtered dope and operable to produce a signal indicative of the flow of filtered dope through the first (15a) and second (15b) filters, and means (33) responsive to the signal generated by the sensor means (32) which is operable to control the speed of the pump means (20) to maintain a predetermined flow of filtered dope through the first filter assembly. 5. A dope filtration system according to any preceding claim, wherein the first filter assembly (15) comprises a plurality of tubes (16) having a filter media made of sintered metal fibres matting mounted in a sealed vessel(19) . 6. A dope filtration system according to any preceding claim, wherein the filter media of the first filter assembly (15) have a pore size which will filter out particles in the range of 20μ to 30μ. 7. A dope filtration system according to any preceding claim, wherein the filter media of the final filter assembly (23) has a pore size which will filter out particles of between 70μ to 80μ. 8. A dope filtration system according to any preceding claim wherein there is one or more intermediate filter assembly (21) with a filter media having a pore size which will filter out particles in the range of 30μ to 40μ. 9. A dope filtration system according to any preceding claim, wherein the diameter of the spinnerette jet holes (22b) is in the range of 70μ to 80μ, a first of the filter assemblies (15) has a pore size which will filter out particles of between 20μ to 30μ, a second of the filter assemblies (21) downstream of the first filter assembly (15) has a pore size which will filter out particles of between 30μ to 40μ, and a third of the filter assemblies (23) downstream of the second filter assembly (21) has a pore size which will filter out particles less than 80μ. 10. A dope filtration system according to any preceding claim, wherein the first filter assembly (15) constitutes the bulk filter and the other filter assemblies (21, 23) constitute line filters between the bulk filter and the spinning heads (22) .

Inventors: GRAY GARY EDWARD GEORGE, GRAY, GARY, EDWARD, GEORGE
Applicants: COURTAULDS FIBRES HOLDINGS LTD, COURTAULDS FIBRES (HOLDINGS) LIMITED
Application year: 1994
Main classification: D01D1
IPC codes: B01D29, B01D35, B01D36, D01D1, D01F2